From 6f1a60366f2337f55dadb36f7bce269ab88ba01d Mon Sep 17 00:00:00 2001
From: Jakob Stoklund Olesen <jolesen@mozilla.com>
Date: Fri, 4 Nov 2016 14:49:31 -0700
Subject: [PATCH] Create value aliases when necessary.

If a secondary value in the source pattern becomes a primary value in
the destination pattern, it is not possible to overwrite the definition
of the source value.

Instead, change the original source value to an alias to the new promary
value.
---
 filetests/isa/riscv/expand-i32.cton |  3 +++
 lib/cretonne/meta/gen_legalizer.py  | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/filetests/isa/riscv/expand-i32.cton b/filetests/isa/riscv/expand-i32.cton
index b2473aa608..c0ca7be56f 100644
--- a/filetests/isa/riscv/expand-i32.cton
+++ b/filetests/isa/riscv/expand-i32.cton
@@ -16,3 +16,6 @@ ebb0(v1: i32, v2: i32):
 }
 ; check: $v3 = iadd $v1, $v2
 ; check: $(cout=$V) = icmp ult, $v3, $v1
+; It's possible the legalizer will rewrite these value aliases in the future.
+; check: $v4 -> $cout
+; check: return $v3, $v4
diff --git a/lib/cretonne/meta/gen_legalizer.py b/lib/cretonne/meta/gen_legalizer.py
index bc662dca8b..6c154adc56 100644
--- a/lib/cretonne/meta/gen_legalizer.py
+++ b/lib/cretonne/meta/gen_legalizer.py
@@ -106,6 +106,7 @@ def emit_dst_inst(node, fmt):
     # type: (Def, Formatter) -> None
     exact_replace = False
     replaced_inst = None  # type: str
+    fixup_first_result = False
     if len(node.defs) == 0:
         # This node doesn't define any values, so just insert the new
         # instruction.
@@ -125,6 +126,7 @@ def emit_dst_inst(node, fmt):
             # Insert a new instruction since its primary def doesn't match the
             # src.
             builder = 'let {} = dfg.ins(pos)'.format(wrap_tup(node.defs))
+            fixup_first_result = node.defs[0].is_output()
 
     fmt.line('{}.{};'.format(builder, node.expr.rust_builder()))
 
@@ -139,6 +141,14 @@ def emit_dst_inst(node, fmt):
     if exact_replace:
         fmt.comment('exactreplacement')
 
+    # Fix up any output vars.
+    if fixup_first_result:
+        # The first result of the instruction just inserted is an output var,
+        # but it was not a primary result in the source pattern.
+        # We need to change the original value to an alias of the primary one
+        # we just inserted.
+        fmt.line('dfg.change_to_alias(src_{0}, {0});'.format(node.defs[0]))
+
 
 def gen_xform(xform, fmt):
     # type: (XForm, Formatter) -> None